The method used for measuring time in a deflector system has been used many years. Almost no modifications in the algorithm have been done so far. Only the pattern used for different kinds of calibrations has been modified during the years. Today we have an experimental verified repeatability of the method in the range of 10-15 nm over a surface of 800×800 mm. The 10-15 nm means here the measurement overlay.
One drawback of the method used is that we so far only can measure in the same direction as the micro sweep. In order to measure an X-coordinate we therefore must use special patterns containing 45-degree bars.
The method according to prior art is briefly described, since it is important to understand the present invention.
It is difficult to measure time with high accuracy. If, for example, you want to measure a pulse with the resolution of 1 nanosecond (ns) you need a measurement clock with the frequency of 1 GHz if classical frequency measurement methods are used. In the described prior art system, there is no need to measure a single shot of a pulse. The use a scanning beam while measuring will get several one-dimensional images of a bar or several bars, as an example. Only the “average” position of an edge or the CD of a bar is interesting. The measurement system will only give an average result together with its sigma. It is important to remember that the measurement system is good enough if this sigma is lower that the natural noise in the system. This natural noise can be summarized to be laser noise, electronically noise and mechanical noise. The noise from the measurement system itself can be calculated theoretically or verified in practice with a known reference signal. It is also possible to get a figure of the measurement system noise by simulation. The measurement of the position of the bar or the CD will therefore contain the error:
      Error    tot    =                              (                      Error            natural                    )                2            +                        (                      Error            measurement                    )                2            
When we measure time we use a so-called random phase method. What this means is that the measurement unit it-self is completely un-correlated in phase to the signal we want to measure. Due to the fact that the signal phase is random relative the measurement clock phase we can use a measurement clock frequency that is much lower and use an “averaging” effect instead to achieve the accuracy.
In FIG. 1 the measurement clock phase is shown relative the reference signal (SOS). Please note that the input signal (the bar) is synchronized with the reference signal since it is generated from the micro sweep itself. The upper row of clocks in FIG. 1 is the ruler marked in measuring clock increments. What we are after is where the positive going edge 10 of the input signal is relative our reference signal. Of course we also are interested of the negative going edge 11. But the same method may be used to find the position of any edge.
Let us call the period time of the measurement clock tm. Since the input signal is a result from the micro sweep we also know exactly the relationship between the pixel clock period in time and what that corresponds to in nanometers. Here we introduce tp for the pixel clock period in nanoseconds. We also call the pixel clock period in nanometers for pp. The scaling expression can therefore be expressed as:
      pm    ⁡          (      nm      )        =                    pp        ⁡                  (          nm          )                            tp        ⁡                  (          ns          )                      ·          tm      ⁡              (        ns        )            pm is what each measurement clock period corresponds to in nanometers. From FIG. 1 we can see that the approximate position of the first edge, denoted 10, is 8 pixel clocks. Please note that by doing only one measurement i.e. using one of the six measurements 1-6 we can se that the edge is within the range of 8-10 measurement clocks. The accuracy is in other words 2*tm. Using above scaling expression this can be expressed in nanometers too.
In the following some realistic numbers are introduced.tm=(1/40)=25 ns.tp=(1/46,7)=21.413 ns.pp=250 nm.
This results in that the pm=291.86 nm.
If we now count measurement clock ticks by resetting a counter by the reference signal we see that we only will count 8 or 9 ticks. No other count is possible in this example. The edge position relative the phase of the measurement clock will in this way be rectangular distributed inside tm. The average position can therefore be calculated just by adding counts from several measurements together and divide this number with number of measurements. In this example we get (8+8+8+8+9+9)/6=8.33 counts as an average value. So an estimation of the position of the edge can be calculated to be:8.33×291.86=2432 nm.
Now it is not enough just to use 6 measurements as in this example. Normally you use several thousands of measurements. (In the detailed description, the three sigma of the average value is described from a theoretical point of view.)